1. Field of the Invention
The present invention relates generally to the field of power generation, and industrial boiler design. In particular, the present invention provides a new and useful system and method which employs a pre-economizer to achieve an increase in power-generating capability in a pulp mill setting.
2. Description of Related Art
Pulp and paper mills are constantly seeking ways to increase the power output and efficiency of steam generators. Raukola et al., in a technical paper entitled “Increasing Power Generation with Black Liquor Recovery Boiler” presented at the 2002 TAPPI Fall Conference & Trade Fair, describe several approaches. These include: increased dry solids content of the black liquor to increase boiler efficiency; air preheating with extraction steam from the steam turbine; taking lower pressure sootblowing steam from extraction steam from the steam turbine, rather than from after the primary superheater, in order to extract more useful work from the steam; in back-pressure steam turbine installations, not throttling the back-pressure steam in order to increase feedwater temperature; employing high-pressure feed water preheaters using extraction steam from the steam turbine; increasing main steam temperature and pressure (noting, however, that corrosion of the furnace walls and in the superheater area are the biggest concerns related to this approach); providing a reheater arrangement where the main steam, after expanding through the turbine, is sent back to the boiler to be superheated again before the next turbine stage; employing a condensing steam turbine instead of a back-pressure steam turbine; and employing heat recovery after the electrostatic precipitator to replace back-pressure steam used normally for preheating and thus releases steam to be used for power generation with the condensing turbine.
Typically, pulp mills use 40 psig to 70 psig of steam to heat water in their deaerator tanks. This heats the feedwater to temperatures of 290° F. to 315° F. Steam used to heat the deaerator tank is extracted from a turbine and reduces power generation by removing turbine steam available for the condenser. Since pulp mills have a considerable volume of make-up process water, this can be a substantial steam load.
Increasing the feedwater temperature by either increasing the DA pressure or adding a feedwater heater will increase the boiler's steam generation but will concomitantly increase the boiler's exit gas temperature and reduce boiler efficiency. Increasing the feedwater heating will also increase the turbine extraction and reduce power generation.
As shown in FIG. 1, pulp mills have historically accepted exit gas temperatures of 350° F. to 420° F. as dictated by the feedwater temperature.
As shown in FIG. 2, one proposed alternative solution has been to perform feedwater heating of the make-up water in a pre-economizer before delivery to the DA tank. This is problematic, however, since the water still has air and other corrosive gases and volatile compounds in it. Also pH control is usually performed after the DA tank since the DA tank will drive off the volatiles used for pH control. Further, the tubes in the pre-economizer are subject to corrosion as a result of moisture condensing on the outside of the tube as a result of the low water temperatures of the makeup water. This arrangement was shown in the paper “A Cost Benefit Assessment of Biomass Gasification Power generation in the Pulp & Paper Industry,” 8 Oct. 2003, Larson. However, the paper fails to teach how the authors would overcome the difficulties of corrosion in the economizer before the DA tank.